# std::ranges::copy_backward, std::ranges::copy_backward_result

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::bidirectional_iterator I1, std::sentinel_for<I1> S1, std::bidirectional_iterator I2 > requires std::indirectly_copyable<I1, I2> constexpr copy_backward_result<I1, I2> copy_backward( I1 first, S1 last, I2 result ); | (1) | (since C++20)
template< ranges::bidirectional_range R, std::bidirectional_iterator I > requires std::indirectly_copyable<ranges::iterator_t<R>, I> constexpr copy_backward_result<ranges::borrowed_iterator_t<R>, I> copy_backward( R&& r, I result ); | (2) | (since C++20)
Helper types |  |
template< class I1, class I2 > using copy_backward_result = ranges::in_out_result<I1, I2>; | (3) | (since C++20)

1) Copies the elements from the range, defined by `[`first`,` last`)`, to another range `[`result - N`,` result`)`, where N = ranges::distance(first, last). The elements are copied in reverse order (the last element is copied first), but their relative order is preserved. The behavior is undefined if result is within `(first, last]`. In such a case std::ranges::copy can be used instead.
2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first, and ranges::end(r) as last.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first | - | the beginning of the range of elements to copy from
last | - | the end of the range of elements to copy from
r | - | the range of the elements to copy from
result | - | the end of the destination range

### Return value

{last, result - N}

### Complexity

Exactly N assignments.

### Notes

When copying overlapping ranges, `ranges::copy` is appropriate when copying to the left (beginning of the destination range is outside the source range) while `ranges::copy_backward` is appropriate when copying to the right (end of the destination range is outside the source range).

### Possible implementation

    struct copy_backward_fn
    {
        template<std::bidirectional_iterator I1, std::sentinel_for<I1> S1,
                 std::bidirectional_iterator I2>
        requires std::indirectly_copyable<I1, I2>
        constexpr ranges::copy_backward_result<I1, I2>
            operator()(I1 first, S1 last, I2 result) const
        {
            I1 last1 {ranges::next(first, std::move(last))};
            for (I1 i {last1}; i != first;)
                *--result = *--i;
            return {std::move(last1), std::move(result)};
        }

        template<ranges::bidirectional_range R, std::bidirectional_iterator I>
        requires std::indirectly_copyable<ranges::iterator_t<R>, I>
        constexpr ranges::copy_backward_result<ranges::borrowed_iterator_t<R>, I>
            operator()(R&& r, I result) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), std::move(result));
        }
    };

    inline constexpr copy_backward_fn copy_backward{};

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <ranges>
    #include <string_view>
    #include <vector>

    void print(std::string_view rem, std::ranges::forward_range auto const& r)
    {
        for (std::cout << rem << ": "; auto const& elem : r)
            std::cout << elem << ' ';
        std::cout << '\n';
    }

    int main()
    {
        const auto src = {1, 2, 3, 4};
        print("src", src);

        std::vector<int> dst(src.size() + 2);
        std::ranges::copy_backward(src, dst.end());
        print("dst", dst);

        std::ranges::fill(dst, 0);
        const auto [in, out] =
            std::ranges::copy_backward(src.begin(), src.end() - 2, dst.end());
        print("dst", dst);

        std::cout
            << "(in - src.begin) == " << std::distance(src.begin(), in) << '\n'
            << "(out - dst.begin) == " << std::distance(dst.begin(), out) << '\n';
    }

Output:

    src: 1 2 3 4
    dst: 0 0 1 2 3 4
    dst: 0 0 0 0 1 2
    (in - src.begin) == 2
    (out - dst.begin) == 4

### See also

ranges::copyranges::copy_if (C++20)(C++20) | copies a range of elements to a new location (algorithm function object)
ranges::copy_n (C++20) | copies a number of elements to a new location (algorithm function object)
ranges::remove_copyranges::remove_copy_if (C++20)(C++20) | copies a range of elements omitting those that satisfy specific criteria (algorithm function object)
ranges::replace_copyranges::replace_copy_if (C++20)(C++20) | copies a range, replacing elements satisfying specific criteria with another value (algorithm function object)
ranges::reverse_copy (C++20) | creates a copy of a range that is reversed (algorithm function object)
ranges::rotate_copy (C++20) | copies and rotate a range of elements (algorithm function object)
ranges::unique_copy (C++20) | creates a copy of some range of elements that contains no consecutive duplicates (algorithm function object)
ranges::move (C++20) | moves a range of elements to a new location (algorithm function object)
ranges::move_backward (C++20) | moves a range of elements to a new location in backwards order (algorithm function object)
copy_backward | copies a range of elements in backwards order (function template)

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